Information storage devices are used to retrieve and/or store data in computers and other consumer electronics devices. A magnetic hard disk drive is an example of an information storage device that includes one or more heads that can both read and write, but other information storage devices also include heads—sometimes including heads that cannot write.
In a modern magnetic hard disk drive device, each head is a sub-component of a head-gimbal assembly (HGA) that typically includes a laminated flexure to carry the electrical signals to and from the head. The HGA, in turn, is a sub-component of a head-stack assembly (HSA) that typically includes a plurality of HGAs, an actuator, and a flexible printed circuit (FPC). The plurality of HGAs are attached to various arms of the actuator.
Modern laminated flexures typically include flexure conductive traces that are isolated from a flexure structural layer by a flexure dielectric layer. So that the signals from/to the head can reach the FPC on the actuator body, each HGA flexure includes a flexure tail that extends away from the head along a corresponding actuator arm and ultimately attaches to the FPC adjacent the actuator body. That is, the flexure includes flexure traces that extend from adjacent the head and continue along the flexure tail to flexure electrical connection points adjacent the FPC.
The FPC includes conductive electrical terminals that correspond to the electrical connection points of the flexure tail, and FPC conductive traces that lead from such terminals to a pre-amplifier chip. The FPC conductive traces are typically separated from an FPC stiffener by an FPC dielectric layer. The FPC may also include an FPC cover layer over the FPC conductive traces, the FPC cover layer having a window to allow electrical conduction to the pre-amplifier chip and access to the FPC terminals. The FPC conductive traces may have different widths, for example so that they can match different desired electrical impedances. To facilitate electrical connection of the flexure conductive traces to the FPC conductive electrical terminals during the HSA manufacturing process, the flexure tails must first be properly positioned relative to the FPC, so that the flexure conductive traces are aligned with the FPC conductive electrical terminals. Then the flexure tails must be held or constrained against the FPC conductive electrical terminals while the aforementioned electrical connections are made (e.g. by ultrasonic bonding, solder jet bonding, or solder bump reflow).
However, recently for some disk drive products, the aforementioned electrical connections may employ a type of anisotropic conductive film (ACF) bonding. An anisotropic conductive film is typically an adhesive doped with conductive beads or cylindrical particles of uniform or similar diameter or size. As the doped adhesive is compressed and cured, it is heated and squeezed between the surfaces to be bonded with sufficient uniform pressure that a single layer of the conductive beads makes contact with both surfaces to be bonded. In this way, the thickness of the adhesive layer between the bonded surfaces becomes approximately equal to the size of the conductive beads after those are compressed (i.e. the adhesive layer thickness is preferably less than the original undeformed size of the conductive beads). The cured adhesive film may conduct electricity via the contacting beads in a direction normal to the bonded surfaces (though may not necessarily conduct electricity parallel to the bonded surfaces, since the beads may not touch each other laterally—though axially each bead is forced to contact both of the surfaces to be bonded—hence the term “anisotropic”).
Maintaining sufficiently uniform temperature and pressure during adhesive curing, such that a single layer of conductive beads in an ACF makes contact with both opposing surfaces to be bonded and curing is acceptably uniform, may be achievable for existing HGA designs using a tool that presses only upon a single bond pad. However, in a high-volume manufacturing environment like that necessitated by the very competitive information storage device industry, there is a practical requirement for fast, cost-effective, and robust bonding of many bond pads simultaneously; bonding one bond pad at a time simply takes too much time.
Accordingly, there is a need in the art for an improved FPC design that may facilitate a more uniform bonding temperature to groups of bond pads, to more quickly accomplish reliable electrical connection of the flexure conductive traces to FPC conductive electrical terminals (e.g. by ACF or by any other bonding method that benefits from a more uniform temperature) during HSA manufacture.